Aerials

ABSTRACT

This disclosure relates to off-set or asymmetric aerial systems in which the main reflector which radiates into or receives from space a beam of energy is disposed to one side of the axis of the geometric body of which the main reflector forms part. The aerial system comprises a feed, a sub-reflector and a main relfector, the feed is displaced from the axis of the main reflector and the axis of the sub-reflector is transverse to the axis of the main reflector. This reduces the phenomena known as &#39;&#39;&#39;&#39;bore-sight jitter&#39;&#39;&#39;&#39; and &#39;&#39;&#39;&#39;frequency sensitive squint.

United States Patent Graham Feb. 12, 1974 [5 AERIALS PrimaryExaminer-Maynard R. Wilbur [76] Inventor: Ralph Graham, Century Works,Assistant Exammer' B'chard Berger Lewisham London, England Attorney,Agent, or Flrm-Karl W. Flocks [22] Filed: Dec. 31, 1968 21 Appl. No.:790,507 [57] ABSTRACT This disclosure relates to off-set or asymmetricaerial [30] Foreign Application priority Data systems in which the mainreflector which radiates into or receives from space a beam of energy isdis- Jan. 2, 1968 Great Britain 00274/68 posed to one side of the axisof the geometric y of [52] U S Cl 343/781 343/837 which the mainreflector forms part. The aerial system [51] In} .0 "6m l9/ comprises afeed, a sub-reflector and a main relfector,

the feed is displaced from the axis of the main reflec- [58] Field ofSeal-chm" 343/781 781 782 A tor and the axis of the sub-reflector istransverse to the [56] References Cited axis of the main reflector. Thisreduces the phenomena known as bore-sight jitter" and frequency sensi-UNITED STATES PATENTS five squint" 3,332,083 7/1967 Broussaud 343/837 XM/l/A/ REAR/'07? 5 Claims, 4 Drawing Figures SHEET 1 OF 2 FIG. I PRIORART FIG.2 PRIOR ART PAIENTEUFEBIZIQM 3,792,480

' .SHEET 2 HF 2 Flea FICA

AERIALS BACKGROUND OF THE INVENTION This invention relates to precisiontracking aerials, particularly the microwave aerials of the type using aconcave collimating reflector.

In particular, it relates to the class of reflector aerials known asoff-set or asymmetric reflector aerials, i.e., those in which thereflector lies to one side of the axis.

This type is commonly used in radar and communication systems since itenables the feed system to illuminate the whole of the reflector and thewhole of the energy thus reflected can be radiated from the aerialwithout shadowing or obscuration by any part of the feed system. Thisinvenntion is partiuclarly concerned with the arrangement in which anoff-set sub reflector is used in conjunction with a main reflector. Sucha system is known in which the beam from the main reflector is displacedfrom the'axis of the said reflector and the focus of the main reflectorand also the feed are located on the axis of the main reflector.

This type of off-set aerial suffers from a disadvantage compared with asymmetrical aerial, i.e., one in which the feed is on the centre line ofthe aerial, when used for accurate tracking of a distant source or radartarget. The disadvantage is as follows: when the aerial feeder system isdesigned to receive plane polarised energy the direction of the beamaxis is dependent upon the polarisation of the energy received. This isknown phenomenon and is referred to as bore-sight jitter." When such anaerial is designed to receive circular polarisation the direction of theradar axis is dependent upon the frequency of operation. This is also aknown phenomenon and is referred to as frequency sensitive squint. Bothof these effects are due to thefact that when the electric and magneticfieldsemanating from thefeed have been reflected by the main reflectorthey are curved.

There are several known ways of straightening the field lines of themain reflector, for example, by using a specially designed feeder or byusing a reflector composed of accurately parallel wires or strips, butthese tend to have disadvantages, for example, they are difficult tomanufacture or they only work with one sort of polarisation.

BRIEF SUMMARY OF INVENTION We have found that the boresight jitter andfrequency squint can be substantially eliminated by adjusting thepostions of the sub reflector and feed relative to the main reflector.

Accordingly, the present invention provides an aerial system comprisinga collimating main reflector, a sub reflector and a feed positionedrelative to one another so that energy passing beween the feed and themain reflector is reflected by the sub reflector and outwardly of themain reflector comprises a collimated beam having its axis offset fromthe axis of the main reflector, the feed being displaced from and theaxis of the sub reflector being positioned transversely to the axis ofthe main reflector by such amounts as to substantially eliminateboresight jitter and frequency sensitive squint.

Preferably, the sub reflector and main reflector are part hyperboloidand part paraboloid respectively but may be any suitable coacting pairof surfaces.-

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a known form ofoff-set aerial system,

FIG. 2 shows the curved electric and magnetic fields emanating from themain reflector of FIG. 1.,

FIG. 3 shows diagrammetically an aerial arrangement, and

FIG. 4 shows resultant field lines produced by such an aerial.

The known off-set aerial system shown in FIG. 1 comprises a feed 1 whichdirects energy on to a sub reflector 2 where it is reflected to a mainreflector 3 to produce the emergent main beam of energy 4. The axis ofthe main reflector 3 is shown at 5 and it will be seen that the beam 4is displaced from the axis 5 and that the focus 6 of the main reflector3 and also the feed 1 are located on the axis 5. g

The disadvantages of this known arrangement are discussed above andillustrated in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An improved system according tothe present invention is shown in FIG. 3. This system can, in the usualway, be used both to receive and to radiate energy.

In this arrangement the main reflector 10 is a part paraboloid concavecollimating reflector and the final offset collimated beam 11 leavingthis reflector has an axis 12 parallel to the axis 13 of the mainreflector. The feed 14 is effectively a point source radiating adivergent beam 15 which is reflected from the surface of a parthyperboloid sub reflector 16 onto the main reflector to produce the beam11. The feed 14 is located on the axis 17 of the hyperboloid, the latterhaving one of its foci coinciding with the feed and its other focuscoinciding with the focus 18 of the main reflector. It will be ssen thatin this arrangement, as compared with the arrangement of FIG. 1, the subreflector l6 and the feed 14 have been rotated about the focus 18 sothat the feed is displaced from and the axis of the sub reflector istransverse to the axis 13 of the main reflector. This necessitatesextending the upper edge of the hyperboloid surface so that a differentportion of the sub reflector 16 is used. As seen in FIG. 4, thedistortion produced in the field lines shown in FIG. 2 has beensubstantially eliminated.

In an aerial according to the invention the following parameters may beadjusted to achieve the desired performance:

a. Focal length of the main reflector b. Eccentricity of the subreflector 0. Region of the main reflector to be used d. Distance of thefeed from the focus e. Displacement of the feed from the axis f.Orientation of the feed g. Type of feed The sub reflector 16 and mainreflector 10 would normally be hyperbolic and parabolic respectively andonly this type is described by way of example, however, this is notessential and any suitably coacting pair of surfaces could be used.

In general terms the values of these parameters are chosen from thefollowing considerations:

a. Increasing the focal length of the main reflector reduces thepolarisation distortion but is wasteful of space, and tends to increasethe weight and inertia of the system.

b. Increasing the eccentricity of the sub reflector reduces thedistortion but also reduces the capability of scanning the beam bymovement of the sub reflector.

c. Reducing the operational region of main reflector reduces thedistortion but leads to poor space utilization.

d. Increasing the distance of the feed from the focus reduces thedistortion and increases the capability of scanning the beam by movementof the sub reflector.

e. The displacement of the feed from the axis can be optimised to givethe minimum distortion consistent with the other chosen parameters.

f. The feed is normally orientated so that the energy is directedtowards the centre of the reflectors.

g. A horn feed is normally used since it may be designed to illuminatethe reflectors efficiently.

An example of an aerial arrangement according to the invention withminimal polarisation distortion has the following parameters:

a. Focal length of main reflector 100 units b. Eccentricity of subreflector 1.85

c. Region of main reflector extending from 40 to 150 units from the axis13 d. Distance of feed from the focus of the main reflector 100 units e.Displacement of feed from the main reflector axis 4 rotation about mainreflector focus f. Orientation of feed beam axis of feed inclined 15with respect to the sub reflector axis g. Type of feed horn.

What I claim is:

1. An aerial system comprising a collimating main reflector, a subreflector and a feed positioned relative to one another so that energypassing between the feed and the main reflector is reflected by the subreflector and outwardly of the main reflector comprises a collimatedbeam, having its axis offset from the axis of the main reflector, thefeed being displaced from and the axis of the sub reflector beingpositioned transversely to the axis of the main reflector by suchamounts as to substantially eliminate boresight jitter, and frequencysensitive squint.

2. An aerial system according to claim 1 in which the sub reflector andthe main reflector are part hyperboloid and part paraboloidrespectively.

3. An aerial system according to claim 2 in which the sub reflector hasone focus coinciding with the focus of the main reflector and a secondfocus coinciding with the location of the feed.

4. An aerial system according to claim 3 in which the focal length ofthe main reflector is units, the eccentricity of the sub reflector is1.85, the region of the main reflector extends from 40 to units from itsaxis measured in a direction normal thereto, the distance of the feedfrom the focus of the main reflector is 100 units, the feed is displacedfrom the axis of the main reflector by a 4 rotation about the mainreflector focus and the feed is orientated to have a beam axis in clinedat 15 with respect to the sub reflector axis.

5. An aerial system according to claim 4 in which the feed is a horn.

1. An aerial system comprising a collimating main reflector, a subreflector and a feed positioned relative to one another so that energypassing between the feed and the main reflector is reflected by the subreflector and outwardly of the main reflector comprises a collimatedbeam, having its axis offset from the axis of the main reflector, thefeed being displaced from and the axis of the sub reflector beingpositioned transversely to the axis of the main reflector by suchamounts as to substantially eliminate boresight jitter, and frequencysensitive squint.
 2. An aerial system according to claim 1 in which thesub reflector and the main reflector are part hyperboloid and partparaboloid respectively.
 3. An aerial system according to claim 2 inwhich the sub reflector has one focus coinciding with the focus of themain reflector and a second focus coinciding with the location of thefeed.
 4. An aerial system according to claim 3 in which the focal lengthof the main reflector is 100 units, the eccentricity of the subreflector is 1.85, the region of the main reflector extends from 40 to150 units from its axis measured in a direction normal thereto, thedistance of the feed from the focus of the main reflector is 100 units,the feed is displaced from the axis of the main reflector by a 4*rotation about the main reflector focus and the feed is orientated tohave a beam axis inclined at 15* with respect to the sub reflector axis.5. An aerial system according to claim 4 in which the feed is a horn.